The Navstar/Global Positioning System (“GPS”) comprises a constellation of satellites, control stations, and user stations (receivers) intended to support user navigation and time distribution on a world-wide basis. Each individual satellite transmits precisely-timed ranging signals as well as its ephemeris data that describes its own trajectory through space as a function of time. A user station can determine its own position and clock offset, relative to GPS system time, by tracking the signals from multiple satellites, determining so-called “pseudoranges” to these satellites, demodulating the data transmitted by these satellites, and solving for its own position and clock offset given the pseudoranges and satellite ephemeris data. The clock offset relative to Universal Coordinated Time can be determined by applying an additional offset parameter which is transmitted by the satellites. User station velocity can be determined by differentiating position estimates over time, or by direct calculation from Doppler measurements on the satellite downlink signals. The Doppler measurements may be based on ranging codes transmitted as a pseudo-random sequence transmitted on a carrier signal. The GPS is maintained and operated by the United States of America. The GLONASS system, maintained and operated by the Russian Federation, is similar in many respects to GPS.
The Global Navigation Satellite System (GNSS) is a loosely-defined super-set of systems, including GPS, GLONASS, and other existing and planned systems, intended to support navigation and time distribution.
The GPS, GLONASS, and GNSS systems are typically subject to a combination of impairments which limit the accuracy of user navigation. These include slowly-varying impairments such as reporting errors in the satellite ephemeris, satellite clock drift, and atmospheric propagation factors, and rapidly-varying impairments such as receiver measurement noise. The slowly-varying impairments tend to be common to a group of stations operating in a local area (10 s or 100 s of miles in extent). These slowly-varying impairments can be estimated by a “base station” at a surveyed location and transmitted to user stations at non-surveyed locations (or in motion), allowing the associated errors experienced by the user stations to be reduced. Such cooperative systems are typically called “differential systems,” and can be referred by various names such as DGPS and DGNSS. There are a wide variety of differential systems incorporating a variety of techniques.
In one type of differential system, the base station calculates an expected range to each satellite (at an instant of time) based on its surveyed location and the ephemeris data for each satellite. The base station compares this expected range to a measured range (based on the ranging codes sent by a satellite) at the same instant of time. The difference contains the slowly-varying impairments as well as the rapidly-varying impairments. The difference is reported to the user station(s) over a datalink (possibly along with other information). The difference is then applied at the user station to correct its own pseudorange observations of the satellites prior to calculating its position and clock offset. This type of differential system is sometimes called a “corrected-differential,” “range-domain,” or “observation-domain” system since it transfers corrections associated with the range or pseudorange observations made at the various stations. The required data rate of the datalink is typically dictated, in large measure, by the need to transfer a separate observation or set of observations for each of several satellites at a specified rate or set of rates. The number of satellites typically varies from 4 to 12 (although more satellites could be reported in the future as additional satellite constellations are deployed and integrated into the GNSS). The observations might typically include, as an example, the arrival time differences (i.e., observed minus expected arrival times), or the equivalent in range offset, for each of the ranging codes transmitted by the satellites in view of the base station. In addition, the observations can include the integrated carrier phases of the carrier signals upon which the ranging codes are modulated, or other information relating to carrier phase measurements and observations. A DGNSS system that transmits and uses information based on carrier phase measurements, in order to improve user station positioning and navigation performance in real time, is typically referred as a real-time kinematic (RTK) system. RTK systems offer substantially better navigation performance (lower navigation and position errors) than non-RTK systems because of a higher frequency, but as a result of the higher frequency typically involve substantially higher data rates, longer initialization times, greater computational burdens on the part of the user station (either within the GNSS receiver or an external processor), and a smaller maximum achievable separation distance between a base station and rover.
Several internationally-recognized standards exist for various types of differential systems (e.g., RTCM SC-104, RTCA DO-217, RTCA DO-229). These standards describe, among other things, the datalink message formats used to transfer information. Several manufacturers of GNSS equipment have developed their own proprietary standards for implementing differential GNSS systems. These standards typically employ observation-domain techniques.
In another type of differential system, the base station determines its position based on the observations it can make (and the ephemeris data) and compares this position to its surveyed position. The difference between the computed and surveyed positions is reported to the user station(s) over a datalink (possibly along with other information intended to add features to the differential system). A user station calculates its own position based on the observations it can make (and the ephemeris data) and corrects this position with the difference reported by the base station. This type of differential system is sometimes called a “differenced-differential,” “position-domain,” “navigation-domain,” or “solution-domain” system since it transfers corrections associated with the 3D navigation solution generated by a typical GNSS receiver. The required data rate of the datalink is typically dictated, in large measure, by the need to transfer a 3D position of the base station (the reference station) at a specified rate. Since there are only three values transferred at high rate (e.g., the computed offset of the base station from its surveyed location in an Earth-centered, Earth-fixed coordinate frame), this type of differential system typically requires a lower data rate than an “observation-domain” system. However, good performance is typically obtained only if the base station and user station rely on the same set of satellites to compute their respective locations. When different sets of satellites are relied-upon, then the difference calculated by the base station may reflect a significant contribution by a satellite not relied upon by the user station or may fail to reflect a significant contribution by a satellite only relied-upon by the user station. Thus, the difference calculated by the base station may not accurately reflect the actual difference of the user station. A well-designed navigation-domain differential system can achieve roughly the same level of performance as a non-RTK observation-domain system. There is, however, no equivalent of RTK performance in navigation-domain differential systems (although the individual stations can smooth-out their navigation solutions using carrier phase tracking techniques). It is also possible for two user stations in an RTK differential system to exchange information regarding their navigation-domain positions or trajectories, in order to develop an accurate relative baseline between them. However, this is not a navigation-domain differential system; it is merely an exchange of positioning data.
Many commercially-available GNSS receivers have the ability to either generate differential correction data and observations (under certain operational constraints and given certain data, such as a surveyed location), or apply differential correction data in order to determine a relative position or a differentially-corrected absolute position, or both. However, many commercially-available GNSS receivers, able to generate or apply differential correction data and observations, have distinct and disjoint modes of operation: In one mode that assumes a fixed or non-moving dynamic at a specified location (e.g., a “reference station” mode), they can generate differential correction data and observations. In another mode (e.g., a “rover mode” that allows movement), they can apply differential corrections and observation data to generate a relative baseline and/or a differentially-corrected absolute position.
Most DGNSS systems rely on a fixed base station. Some commercially-available GNSS receivers can be operated as a component of a differential base station while they are in motion, without declaring a surveyed location. However, many commercially-available GNSS receivers, when configured to operate as a component of a differential base station, assume that the receiver is at a fixed position relative to the Earth. Such a receiver may require that a survey position be specified before it will act as a base station, and may then report zero velocity (relative to the Earth) even if it is moving. Such a receiver may also fail to operate correctly if it is moving, generating integrity alarms or false data (which may introduce errors at the user stations) if the position offset from its surveyed location, or its velocity, exceed certain limits.
In some cases it is necessary to determine the relative position of two stations that are in motion (or could be in motion), or for which no surveyed location is available for either station. The relative position of two stations is sometimes referred as the “baseline” between them. In such cases, the observations made at the two stations (or the positions calculated from such observations) can be compared to determine a relative position or baseline. This data can be extracted from typical GNSS receivers operated in a “rover mode,” regardless of the receivers' operational role in a larger system. The distinction between a base station and a rover station is less clear in such cases, although it may be possible to differentiate based on operational considerations.
In some cases, it is desired to determine a relative position (a baseline) between two platforms that could be moving with respect to the Earth, using GNSS receivers mounted on the platforms, where the GNSS receivers employed cannot effectively operate in a base station mode (e.g., as a “reference station”) if they are moving. This can be achieved by operating both GNSS receivers as rovers, collecting observation data and other data such as ephemeris data from the two receivers, bringing the observation data and other data to a common location (for example, one of the platforms supporting one of the two receivers), and calculating the relative position or baseline in a separate computer. However, this requires the development of appropriate datalink protocols and algorithms and the availability of a separate computer with sufficient computational ability to perform the necessary calculations in the required timeframe.
It would be desirable to determine a relative baseline between two platforms that are moving with respect to the Earth, using GNSS equipment that assumes it is stationary when operated in a reference station mode, and while minimizing the computational burden of associated computer resources.
It would further be desirable to minimize datalink loading for a given level of performance (navigation or position error) associated with the computed relative position or baseline, by combining the concepts of “observation-domain” and “navigation-domain” differential systems.